Where did all our water come from? This article may offer a clue.
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University of Rochester
August 28, 2007
Supersonic 'Rain' Falls on Newborn Star
Forming Solar System Deluged with Oceans of Water
Astronomers at the University of Rochester have discovered five
Earth-oceans' worth of water that has recently fallen into the
planet-forming region around an extremely young, developing star.
Dan Watson, professor of physics and astronomy at the University of
Rochester, believes he and his colleagues are the first to see a
short-lived stage of protoplanetary disk formation, and the manner in
which a planetary system's supply of water arrives from the natal
envelope within which its parent star originally formed.
The findings, published in today's Nature, are the first-ever glimpse of
material directly feeding a protoplanetary disk.
The embryonic star in question, called IRAS 4B, lies in a picturesque
nebula called NGC 1333, about 1000 light years from Earth. It is one of
an initial list of 30 of the youngest "protostars" known, which Watson
and his team examined with the Spitzer Space Telescope's infrared
spectrograph for signs of very dense, warm material at their cores. It
is also the only one of the thirty to show signs of such material,
signaled by the infrared spectrum of water vapor. The watery
characteristics of IRAS4B's infrared spectrum can best be explained by
material falling from the protostar's envelope onto a surrounding, dense
disk. This setup, called by astronomers a "disk-accretion shock," is the
formative mechanism of the disks within which all planetary systems are
thought to originate.
"Icy material from the envelope is in free-fall, reaching supersonic
speeds and crashing into the protoplanetary disk." says Watson. "The ice
vaporizes on impact, and the warm water vapor emits a distinctive
spectrum of infrared light. That light is what we measured. From the
details of the measured spectrum we can tease out the physical details
of this brand-new, pre-planetary disk"
Among the details derived so far are the rate of "rainfall" onto the
disk about 23 Earth masses per year and the characteristics of the
"puddle" on the disk's surface: The surface is 170 degrees Kelvin (153
degrees below zero Fahrenheit), and at that temperature there is about
an Earth's mass worth of material, including enough water to fill
Earth's oceans about five times. The area of the "puddle" is such that,
if circular and centered on the Sun, its perimeter would be just beyond
the orbit of Pluto. Results such as this will help astronomers assess
the early planet-forming potential of IRAS4B's disk, and by inference
learn about the earliest stages of our solar system's life.
There are astro-chemical implications of the observations as well.
"There are lots of primitive icy bodies in our solar system, and the ice
they carry is often thought to descend directly from the interstellar
medium, so that by studying one we could learn about the other," says
Watson. "But in NGC 1333 IRAS 4B's disk, it is clear that the water is
received as vapor and will be re-frozen under different conditions, and
this means that the oxygen and hydrogen chemistry of its disk is reset
from interstellar conditions. It's not getting pristine, interstellar
ice."
Astronomers at the University of Rochester, including Watson and
co-author professor William Forrest, helped design the "eyes" of Spitzer
specifically to look for objects like IRAS4B and its water because such
objects sit in an astronomer's blind spot. Called "Class Zero
Protostars" for their extreme youth, these objects radiate substantial
light only at long infrared wavelengths, which our atmosphere
inconveniently blocks from ground-based telescopes.
When Watson and his team first planned their Spitzer observations, only
50 class-zero protostars were known, and the team selected the 30
brightest. But Watson says that's just the beginning. Astronomers now
know of hundreds of such objects, and Watson expects to have thousands
to investigate in the coming years.
Another characteristic makes the otherwise un-noteworthy IRAS4B a
rarity. It is oriented with its axis pointed almost directly at Earth,
splaying out its entire disk to our view and simplifying the process of
plumbing its secrets. Only a small fraction of the future candidates are
expected be similarly oriented, keeping the search for lots more
"raining protostars" a challenge.
This work was supported in part by NASA through the Spitzer-IRS
Instrument Team, Origins and Astrobiology programs, and by the National
Science and Technology Council of Mexico.
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http://www.rochester.edu/news/show.php?id=2966
--
John #1782
"We should always be disposed to believe that which appears to us to be
white is really black, if the hierarchy of the church so decides."
- Saint Ignatius Loyola (1491-1556) Founder of the Jesuit Order.
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